Dynamic Power Path Management Simplifies Battery Charging from Solar Panels

The Texas Instruments bq2403x family of linear charge regulators were originally designed to charge single-cell Li-ion batteries from an AC adapter or USB port. However, these ICs also lend themselves well to applications powered by solar panels. Solar cells basically comprise a p-n junction in which incident light energy (photons) causes electrons and holes to recombine, generating an electric current. As the p-n junction’s characteristics are similar to those of a diode, the electrical circuit shown in Figure 1 is often used as a simplified model of the cell’s characteristics. Figure 1. Simplified Model of a Solar Cell The current source IPH generates a current proportional to the amount of light falling on the cell. With no load connected, nearly all the current generated flows through diode D1, whose forward voltage determines the solar cell's open-circuit voltage (VOC). This voltage varies somewhat with the exact properties of each type of solar cell, but for most silicon cells, it is in the range 0.5 V to 0.6 V (the normal forward voltage of a p-n junction diode). The parallel resistor RP represents a small leakage current that occurs in practical cells. As the load current increases, more of the current generated by IPH is diverted away from the diode and into the load. For most values of load current, this has only a small effect on the output voltage; there is a small change due to the diode's I-V characteristic, and a small voltage drop due to the series resistor RS (which represents connection losses) but the output voltage remains largely constant. However, at some point, the current flowing through D1 becomes so small that the diode starts to become insufficiently biased, and the voltage across it decreases rapidly with increasing load current. Finally, when all the current generated flows through the load (and none through the diode), the output voltage is zero. This current is known as the solar cell's short-circuit current (ISC) and, together with VOC, is one of the primary parameters defining its operating performance (see Figure 2). bqTINY is a trademark of Texas Instruments. SLUA394–October 2006 Dynamic Power Path Management Simplifies Battery Charging from Solar Panels 1 Submit Documentation Feedback